Document and update the API of the externals.rs module (#812)

* Document and update the API of the `externals.rs` module

This commit ensures that all public methods and items are documented in
the `externals.rs` module, notably all external values that can be
imported and exported in WebAssembly. Along the way this also tidies up
the API and fixes a few bugs:

* `Global::new` now returns a `Result` and fails if the provided value
  does not match the type of the global.
* `Global::set` now returns a `Result` and fails if the global is either
  immutable or the provided value doesn't match the type of the global.
* `Table::new` now fails if the provided initializer does not match the
  element type.
* `Table::get` now returns `Option<Val>` instead of implicitly returning
  null.
* `Table::set` now returns `Result<()>`, returning an error on out of
  bounds or if the input type is of the wrong type.
* `Table::grow` now returns `Result<u32>`, returning the previous number
  of table elements if succesful or an error if the maximum is reached
  or the initializer value is of the wrong type. Additionally a bug was
  fixed here where if the wrong initializer was provided the table would
  be grown still, but initialization would fail.
* `Memory::data` was renamed to `Memory::data_unchecked_mut`.
  Additionally `Memory::data_unchecked` was added. Lots of caveats were
  written down about how using the method can go wrong.
* `Memory::grow` now returns `Result<u32>`, returning an error if growth
  fails or the number of pages previous the growth if successful.

* Run rustfmt

* Fix another test

* Update crates/api/src/externals.rs

Co-Authored-By: Sergei Pepyakin <s.pepyakin@gmail.com>

Co-authored-by: Sergei Pepyakin <s.pepyakin@gmail.com>

crates/api/src/trampoline/table.rs

@@ -13,10 +13,7 @@ pub fn create_handle_with_table(table: &TableType) -> Result<InstanceHandle> {
         maximum: table.limits().max(),
         ty: match table.element() {
             ValType::FuncRef => wasm::TableElementType::Func,
-            _ => match table.element().get_wasmtime_type() {
-                Some(t) => wasm::TableElementType::Val(t),
-                None => bail!("cannot support {:?} as a table element", table.element()),
-            },
+            _ => bail!("cannot support {:?} as a table element", table.element()),
         },
     };
     let tunable = Default::default();